Your body stores nutrients (obviously) so that if you miss a meal you don't expire. So when you're not eating, in what ratio does your body retrieve macronutrients from storage? The ratio is 70% fat, 20% carbohydrates, and 10% protein. (This figure came from one of Jimmy Moore's podcast interviews, which I'm not going to go re-listen to to verify. I think it was Dr. Larry McCleary, however.)
That's pretty much a high-fat, low-carb diet, which also (oddly) seems to have good effects on heart-disease markers.
So it's not too surprising that intermittent fasting should have similar effects:
"For that research, also presented at the New Orleans conference, 30 patients were asked to fast for 24 hours with water only. The scientists used blood tests before and after the fasting period to look at a number of different metabolic markers. Among other changes, they found that levels of human growth hormone, or HGH, surged after fasting — increasing 20 times in men and 13 times in women. The hormone is released by the body in times of starvation to protect lean muscle mass and trigger the body to start burning fat stores.
"'There is a lot more to be done to fill in the research on the biological mechanism,' Dr. Horne said. 'But what it does suggest is that fasting is not a marker for other healthy lifestyle behaviors. It appears to be that fasting is causing some major stress, and the body responds to that by some protective mechanisms that potentially have a beneficial long-term effect on risk of chronic disease.'"
Or it could just be that not eating the Modern American Diet for a while is good for you...
FuturePundit read the same research, and came to the same conclusion that I did:
"The best diet changes to make amount to turning the clock back: Reverse the wheat, vegetable oils, and sweetener consumption increase of the modern age before you start thinking about fasting."